This invention relates to a method and apparatus for the induction heat treatment of conducting workpieces, and more particularly to an apparatus and method for heat treating irregularly shaped workpieces lacking rotational symmetry about a central axis, in which such heat treatment effects the surface of the workpiece substantially uniformly at all points around the perimeter of a workpiece lacking rotational symmetry accomplishing such substantially uniform heat treatment with a close-proximity inductor without requiring excessive current flow and without requiring interruptions of current flow and consequent severe wear to contacts.
Heat treating of metals to improve the properties has been in use for many centuries. In particular, it has long been a part of the art of metallurgy and metal working to apply controlled amounts of heat, in a controlled way (such as in a furnace), and often to control the rate of cooling also (such as by means of rapid cooling in water). Such processes are now known to alter the microscopic structure of the metal or alloy being processed in such a way as to cause beneficial properties to be enhanced. The microstructure of the material may be altered by such heat treatment to create a beneficial structure of defects or chemical bonds; minor constituents of the material may be induced to concentrate preferentially in certain regions under careful heat treatment to enhance the properties of those regions (or of the depleted regions); and many other effects well known in the art and science of metallurgy.
The present invention relates to a method and apparatus for the induction heat treatment of conducting workpieces, typically metals or alloys. Typically such heat treatment is performed as a means for case hardening a region of the surface of such workpiece which will be subject to particular wear when the workpiece is put to use in its intended application. It is desired that such hardening be localized to the region or regions which are subject to wear in the intended use. While heat treatment hardens the region of the workpiece against expected wear, it also tends to make the region of heat treatment brittle. Also, heat treatment tends to distort the workpiece from its original shape, thereby increasing the need for later reworking or causing the workpiece no longer to meet the dimensional tolerances required. Thus, it is important for the engineer to tailor the heat treatment process to achieve the optimal balance of wear resistance, strength and dimensional stability for the intended application. Also, since heat treatment requires energy, time and expense, the engineer is likewise motivated to limit such heat treatments to the regions in which it will be needed and to no greater depth than required.
Induction heat treatment makes use of the basic fact of electricity that a time-varying magnetic field induces an electric field (Faraday's Law). When a conductor, such as the workpiece, is placed in an electric field, currents will flow through the conductor in the direction of the electric field in direct proportion to the strength of the electric field and in inverse proportion to the electrical resistance of the conductor (Coulomb's and Ohm's Law). The resistance of the workpiece to current flow will cause the workpiece to heat in the immediate regions of such current, leading to the desired heat treatment effect.
Thus, the basic structure of induction heat treating is to cause an alternating current to flow through a circuit external to the workpiece. This alternating current is brought into close proximity with the surface of the workpiece to be heat treated carried by a conducting element known to the art as the `inductor`. The flow of alternating current produces an alternating magnetic field in the immediate vicinity of the current (Ampere's Law). The geometry of the alternating current and the workpiece is arranged such that the region of the workpiece to be heat treated is brought into the alternating magnetic field. Thus, heat treating occurs through the mechanism described above.
The depth of the workpiece hardened by such induction heat treating is a function of both the frequency of the alternating current and the power density of current flow induced in the workpiece (with other effects held constant, such as the geometry of the external current flow in the inductor and the workpiece resistivity). Alternating currents with frequencies from approximately 1 kHz to 500 kHz have found application for induction heat treating, with 10 kHz to 50 kHz a more typical range of values. The hardening depth tends to decrease with increasing frequency and with increasing power density (kW/sq. in.) induced in the workpiece.
Induction heat treating has proven itself to be a versatile engineering tool, widely used in many industries to increase the wear resistance of critical components of machinery. While applications in transportation have been dominant, other applications for wear resistance have made use of induction heat treatment as well. Thus, induction heat treatment is a vital and well accepted part of many modern manufacturing processes.
However, there is increasing emphasis on achieving optimum manufacturing efficiency, reducing work-in-process, reducing the need for inventories by just-in-time delivery, and achieving flexible, rapid and reliable throughput at all steps in the manufacturing process. Induction heat treating is one part of the process where such advances can effect both the quality and performance of the final manufactured product, and the efficiency at which it is manufactured.
The present invention describes a method and apparatus for heat treating workpieces having irregular shapes. Such irregular shapes can be induction heat treated in a uniform, economical manner by the invention disclosed herein, without the need for huge currents (as in typical "one-shot" methods), without the need to break the circuit carrying the alternating current during the heat treatment of a workpiece (as in typical "split-inductor" methods) and the resulting high rate of wear to the split-inductor contacts, avoiding the nonuniformities in heat treating irregular parts (such as cams) occurring with the use of circular inductors, and at the same time avoiding the stray heating of adjacent regions.